Get the V8 the difference in gas milage is insignificant.
I had a 2001 Firebird 3.8L as my DD before my Camaro and my 200HP V6 got the same mileage as my 400HP V8.
The only difference is the V6 could run on regular(I used premium anyway) and the 6.2 runs on premium only.
The High Feature V6 needs 89 minimum but runs best on 91 or 94
My G8 loves the extra octane
